"It's natural for human beings to be afraid of new or different
people and strangers, therefore it's our inherent excuse to be racist,
I humbly disagree.
I don't think people are inherently afraid of things that are different. I agree that we are instinctually afraid of things of things like spiders, snakes, tigers, sharks, etc. because the fear of such things has been ingrained in our genes over millenia of evolution. Note: People don't hate such creatures, they're just afraid of them because they're dangerous and that is an important difference.
I don't think people are meant to be racist/sexist/ageist/classist/etc. Such fear/hatred is not the result of genetics but are caused by social constructs created by society.
Also when applied to sexuality if anything we can say with some degree of accuracy that people are naturally attracted to opposites. Men and women aren't naturally afraid of each other. Gynophobia and androphobia only develops later in life due to traumatic events.
Likewise children aren't naturally racist. Children are driven to play with each other and get along easily regardless of race. As they get older they develop beliefs that may cause them to bully, fight and discriminate against other children who dress differently, wear glasses, have different hair, etc. but those beliefs are the result of bad upbringing and certainly not the result of genetics and "natural fears".
Likewise when we apply such ideas to religious discrimination, ageism and classism it becomes obvious that discriminatory behaviour is the result of societal stereotypes against the rich/poor, the young/old and people of different faiths/no faith.
I do think however there might be a natural aversion to people who appear to be confusing with respect to gender/sex. If you see a person who you can't tell whether they are male or female a person's natural response seems to be confusion and avoidance. I don't think its pre-disposed to being fear/hatred (homophobia is still a social construct), but simply that confusion is the result because the viewer is confused as to whether the person is a potential mate or competition.
I'd argue people are more naturally afraid of competition.